The Next Ten Thousand Years: Vision of Man's Future in the Universe - Adrian Berry A dispassionate look into Earth's future (I say "Earth's" since Berry has no problem contemplating the end of human life).

I remember reading it as a child to find out what a truly advanced technological civilization would be getting up to: terraforming Venus and Mars, ripping the Jovian worlds apart to build a Dyson sphere, exploring the nearby galaxy, etc.